1. Field of the Invention
This invention relates to a process suitable for forming a deposited film, in particular a functional film, above all an amorphous or crystalline deposited film to be used for semiconductor devices, photosensitive devices for electrophotography, line sensors for image input, image pick-up devices, photovoltaic devices, etc.
2. Description of the Prior Art
For example, for formation of an amorphous silicon film, an amorphous germanium film, etc. the vacuum deposition method, the plasma CVD method, the CVD method, the reactive sputtering method, the ion plating method, the optical CVD method or the like have been attempted to be utilized, and, in general, the plasma CVD method has widely been used and industrialized.
However, for the deposited film composed of amorphous silicon, amorphous germanium, etc. there is room for further improvement of overall characteristics, that is, electrical or optical characteristic, fatigue characteristic in repeated use, characteristic for use environment, productivity, mass productivity including uniformity, and reproducibility and the like.
The reaction process in formation of an amorphous silicon deposited film, an amorphous germanium deposited film, etc. according to the plasma CVD method generalized in the prior art is considerably complicated as compared with the conventional CVD method and not a few ambiguities existed in its reaction mechanism. Also, there are involved a large number of parameters for formation of such a deposited film (e.g. substrate temperature, flow rates and ratios of gases introduced, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, gas discharging speed, plasma generation system, etc.), and the nature of formed plasma depends on such a large number of parameters and sometimes becomes unstable. This often gives markedly bad influences of the deposited film formed. Besides, the parameters associated with an apparatus must be chosen for each apparatus, and it has been difficult under the present situation to standardize the production conditions. On the other hand, for forming an amorphous silicon film, an amorphous germanium film, etc. having satisfactory electrical, optical, photoconductive or mechanical characteristics for respective uses, it has been deemed best to form such a film according to the plasma CVD method under the present situation.
However, in some applied uses of the deposited film, it is required to sufficiently accomplish enlargement of film area, uniformization of film thickness and film quality, etc. and also to perform a mass production with reproducibility by high speed film formation. Thus, in formation of amorphous silicon deposited film, amorphous germanium deposited films, etc. according to the plasma CVD method, enormous investment for mass production equipment is necessary, and operating conditions for mass production is complicated and it makes the tolerance narrower and makes the regulation of apparatuses delicate. These problems have been pointed out to be improved in the future. On the other hand, in the prior art using the conventional CVD method, high temperature is required and no deposited film having practical characteristics could be obtained.
As described above, in formation of amorphous silicon films, amorphous germanium films, etc., it has earnestly been desired to develop a formation process which can perform mass production by means of a low cost apparatus while maintaining practical characteristics and uniformity. These discussions may also be applicable to other functional films such as silicon nitride films, silicon carbide films, silicon oxide films, germanium nitride films, germanium carbide films, germanium oxide films, etc.